The latest code of WebPageHttpHandler implements IHttpHandler. Is it planned to support IHttpTaskAsyncHandler in 4.5?

We're not planning to support any async functionality in ASP.NET Web Pages. There are two reasons for this at the moment:

For the typical user of ASP.NET Web Pages, async is just too complex a scenario. Web Pages is about having a very shallow learning curve to start building web apps super quickly.

We haven't yet been able to come up with a good way to support this. Just implementing IHttpTaskAsyncHandler is of course not enough - we'd have to come up with a proper programming model to do this. The nature of how CSHTML files are compiled and run doesn't
lend itself well to an async model, where you generally need specific method signatures and using the async and await keywords as well as Task<T>.